The present invention relates to a method of forming a halftone screen, and more particularly to a halftone screen forming method which, when converting a continuous-tone image into a halftone dot image based on electrically generated halftone screen signals, minimum units of halftone dot data determined by screen angles and a dot resolution level to construct a basic periodic portion of the halftone screen signals are successively generated.
Recently, image scanning, reading, and recording apparatuses have been widely used in the field of printing and graphic art for electrically processing image information of originals to produce original film plates in an effort to simplify the entire process and improve the quality of printed images.
The image scanning, reading, and recording apparatuses are basically constructed of an image input unit, an image processing unit, and an image output unit. In the image input unit, an original or subject bearing image information is delivered to a predetermined image reading position. Next the original is scanned by a laser beam or the like, and the image information of the original is converted into an electric signal representative of different intensities of reflected laser light. The photoelectrically converted image information is then processed in the image processing unit for tone conversion, outline emphasis, or the like according to platemaking conditions. The image information which is processed by the image processing unit is thereafter fed to the image output unit in which it is converted again into a light signal such as a laser beam. The light signal is applied to a recording medium of a photosensitive material to record the image thereon. The image on the recording medium is then developed by a developing device, after which the recording medium is used as an original film plate for printing or the like.
Where an original to be printed or reproduced bears a continuous-tone image such as a photograph or a painting, it is necessary to break up the original image into halftone dots in order to reproduce image tone gradations clearly. More specifically, a continuous-tone image is converted into a halftone dot image which is composed of closely spaced dots of different sizes according to gradations of density of the image. One method of breaking up a continuous-tone image into halftone dots is to apply a light signal commensurate with the continuous-tone image to a recording medium through a contact screen having a vignetted dot pattern. The aforesaid image scanning reading and recording apparatuses employ a process for electrically generating halftone screen signals corresponding to such a contact screen and for forming a halftone dot image based on the halftone screen signals.
One conventional method of forming a halftone screen disclosed in Japanese Patent Publication No. 52-49361 will briefly be described below by way of example.
FIG. 1 of the accompanying drawings shows a basic periodic portion 2 of a halftone screen which is electrically formed. The halftone screen is composed of repetitions of one pattern including a dot 4 surrounded by four points A, B, C, D, and the minimum unit of the halftone screen is the basic periodic portion 2. The basic periodic portion 2 is constructed of eight scanning lines S.sub.1 through S.sub.8 arranged side by side in the auxiliary scanning direction Y. Each of the scanning lines S.sub.1 through S.sub.8 forms an element of the basic periodic portion 2 with a voltage signal that varies in the main scanning direction X. For forming the dot 4, for example, the voltages of the scanning lines S.sub.1, S.sub.2, S.sub.4, S.sub.5 passing through the points A through D are high, and the voltage of the scanning line S.sub.3 passing through a point E is low. The voltages of the scanning lines S.sub.1 through S.sub.5 are selected so as to become gradually lower from the points A through D to the point E.
Halftone screen signals which are represented as voltage signals of the respective scanning lines S.sub.1 through S.sub.8 may be produced as analog signals. However, such half-tone screen signals are generally constructed of dot data items as digital signals for simplifying a circuit arrangement required or a signal processing operation. Where the basic periodic portion 2 is expressed by dot data items which are eight digital signals in each of the main and auxiliary scanning directions X, Y, as illustrated in FIG. 2, the basic periodic portion 2 is composed of 64 dot data items.
The basic periodic portion 2 is periodically produced frequently enough to cover an entire scanned region of the original, for thereby producing a halftone screen. The image information which is photoelectrically read by the image input unit of the image scanning, reading, and recording apparatus is reproduced as a halftone dot image on an original film plate based on the dot data items which serve as halftone screen signal s constituting the halftone screen.
When printing or reproducing a multicolor image or a high-quality monochromatic image from the halftone dot image, it is necessary to generate a plurality of halftone screens having different angles of inclination of the dots 4, i.e., different screen angles .theta., and superpose halftone dot images produced respectively by those halftone screens in order to prevent a moire pattern from being produced. The dot data items and their amount of the basic periodic portion 2 vary dependent on the screen angle .theta.. For example, where the screen angle .theta. is 0.degree. or 45.degree., the basic periodic portion 2 may be constructed of a relatively small amount of dot data items. For other screen angles .theta., the amount of dot data items making up the basic periodic portion 2 is considerably increased. The resolution of a half-tone dot image depends upon the data amount. Usually, a plurality of data sets constructing the basic periodic portion 2 and having different resolutions are prepared for one screen angle .theta.. Therefore, the amount of data items that should be available by the image scanning, reading, and recording apparatus is very large. It is quite time-consuming and highly costly to prepare the necessary dot data items. Also capacity of a memory for storing such dot data must be extremely large. Consequently, the apparatus cannot be manufactured inexpensively and compactly.